Paper is generally manufactured by dispersing cellulosic fiber in an aqueous medium and then removing most of the liquid. The paper derives some of its structural integrity from the mechanical interlocking of the cellulosic fibers in the web, but most by far of the paper's strength is derived from hydrogen bonding which links the cellulosic fibers to one another. With paper intended for use as bathroom tissue, the degree of strength imparted by this inter-fiber bonding, while necessary to the utility of the product, can result in a lack of perceived softness that is inimical to consumer acceptance. One common method of increasing the perceived softness and cushion of bathroom tissue is to crepe the paper. Creping is generally effected by fixing the cellulosic web to a Yankee dryer with an adhesive/release agent combination and then scraping the web off the Yankee dryer by means of a creping blade. Creping, by breaking a significant number of inter-fiber bonds in a formed web, adds to and increases the perceived softness of resulting paper product. More specifically, in a typical creping operation, the web is pressed with a smooth roller onto a heated polished metal drying drum, such as the Yankee dryer, while it is partially wet and before final drying. The heat of the drying drum and the pressing with the roller cause the web to adhere to the surface of the cylinder. After the moisture in the web has evaporated, the dried web is creped off the cylinder by a creping blade. However, creping with a conventional creping blade alone may not be sufficient to impart the desired bulk, absorbency, and tactile characteristics.
Conventional creping blades have a flat top surface that is usually ground to give a reasonably smooth, flat surface. The width, W, of a conventional creping blade having a flat top surface is given by: EQU W=T/cos(.theta.)
wherein the blade has a bevel of .theta. degrees and T is the thickness of a cross section of the blade. Very limited literature data indicate that the fineness of a crepe (as measured by crepes/unit length) can be increased by decreasing the thickness, T, of the blade. However, it is well recognized that as the fineness of the crepe increases, the bulk of a resulting sheet decreases. It is unclear whether the creping angle was controlled when thin blades were used. However it is known that if the thin blades flex, the creping angle increases causing the fineness of the crepe to increase. Current understanding is that the fineness of the crepe is inversely related to the thickness of the blade, at a constant creping angle.
Because the flexural rigidity of a thin blade is low, a backing blade must often be used with a thin creping blade to increase its apparent stiffness and prevent it from bending unduly under forces necessary for creping. In order to overcome such a shortcoming, the present invention provides a stepped creping blade, which combines the characteristics of the relatively thin width of a thin creping blade with the stiffness of a thick blade. The stepped creping blade of the present invention does not need to be used with a backing blade because the stepped creping blade has the stiffness of a thick blade.
The present invention is directed to creped paper products such as napkins, towels, or tissues, having increased bulk and increased absorbency which are produced by a process similar to conventional processes, however, a conventional crepe blade is replaced with a stepped creping blade. The stepped creping blade can be manufactured by machining or grinding a step into a conventional blade. In accordance with the present invention, the width of the step's top surface is from 20% to 60% of the total width and the depth of the step is from 30% to 300% of the top surface (0.030 to 0.030). Preferably, the depth of the step is about 0.010 to 0.030 inches and the width of the step's top surface is about 0.010 to 0.040 inches.
In this regard, U.S. Pat. No. 5,520,731 to Esser et al. discloses a doctor blade with a shape similar to a step; however, this doctor blade is used in coating processes as a squeegee to wipe excess ink off an applicator roll. Coating processes are unlike and unrelated to creping processes. In addition, the Esser et al. doctor blade has a chamfered metering surface which is angled so that a portion of the blade's surface, rather than just an edge of the blade, is engaged against an applicator roll for coating a web which is adhered to the applicator roll. Similarly, U.S. Pat. No. 4,184,429 to Widmer discloses a doctor blade much like the Esser et al. blade. U.S. Pat. No. 4,895,071 to Benton discloses a doctor blade having a reduced thickness section for use in the field of gravure printing. The Benton blade has a beveled surface, and is designed to control an amount of ink transferred from a printing surface of a gravure cylinder to a web. The direction of the bevel in the doctor blade surfaces that engage against a cylinder in the Esser et al., Widmer, and Benton patents prevent these blades from being used as a creping blade which produces a creped sheet having increased bulk and increased absorbency. U.S. Pat. No. 5,408,926 to Alder discloses a prior art doctor blade which appears to have a stepped shape. However, the doctor blade of the Alder patent is used in pad transfer printing to scrape excess ink from a working surface. In addition, the blade of the Alder patent is not designed to extend the length of a moving surface, such as a Yankee dryer, in order to crepe a cellulosic web from the moving surface.
U.S. Pat. No. 4,185,399 to Gladish discloses a creping blade having first and second surfaces that are substantially perpendicular to a surface of a Yankee dryer wherein the first surface crepes a web off the Yankee dryer. However, the first surface and the second surface are not parts of a single blade body. In addition, a flow of air is forced between the first surface and the second surface of the Gladish blade.
There is a need for a creping blade and method for producing creped paper products such as napkins, towels or tissues having increased bulk and increased absorbency without increasing the thickness of the paper product but merely by altering the structure of the creping blade itself.